Nand flash memory with vertical cell stack structure and method for manufacturing same

ABSTRACT

Disclosed is a method of manufacturing flash memory with a vertical cell stack structure. The method includes forming source lines in a cell area of a substrate having an ion-implanted well and forming an alignment mark relative to the source lines. The alignment mark is formed in the substrate outside the cell area of the substrate. After formation of the source lines, cell stacking layers are formed. After forming the cell stacking layers, cell pillars in the cell stacking layers are formed at locations relative to the previously formed source lines using the alignment mark to correctly locate the cell pillars.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of application Ser. No.16/521,066 filed Jul. 24, 2019, which is a continuation of Ser. No.13/803,085, filed Mar. 14, 2013, now U.S. Pat. No. 10,403,766, whichclaims the benefit of priority from U.S. Provisional Patent ApplicationNo. 61/733,063 filed Dec. 4, 2012 the disclosure of which is expresslyincorporated herein by reference in its entirety.

FIELD

The present disclosure relates generally to a semiconductor device. Moreparticularly, the present disclosure relates to a nonvolatile memory.

BACKGROUND

Flash memory is a commonly used type of non-volatile memory inwidespread use as storage for consumer electronics and mass storageapplications. Flash memory is pervasive in popular consumer productssuch as digital audio/video players, cell phones and digital cameras,for storing application data and/or media data. Flash memory can furtherbe used as a dedicated storage device, such as a portable flash drivepluggable into a universal serial port (USB) of a personal computer, anda magnetic hard disk drive (HDD) replacement for example. It is wellknown that flash memory is non-volatile, meaning that it retains storeddata in the absence of power, which provides a power savings advantagefor the above mentioned consumer products. Flash memory is suited forsuch applications due to its relatively high density for a given area ofits memory array.

SUMMARY

A broad aspect of the disclosure provides a method of manufacturingflash memory with a vertical cell stack structure, the methodcomprising: forming source lines in a cell area of a substrate having anion-implanted well and forming an alignment mark relative to the sourcelines, the alignment mark being formed in the substrate outside the cellarea of the substrate; after formation of the source lines, forming cellstacking layers; and after forming the cell stacking layers, formingcell pillars in the cell stacking layers at locations relative to thepreviously formed source lines using the alignment mark to correctlylocate the cell pillars.

Another broad aspect of the present disclosure provides a flash memorycomprising: a substrate; a plurality of source lines formed in thesubstrate; a plurality of cell stacking layers formed on the substratecontaining the source lines; a plurality of cell pillars in the cellstacking layers, each cell pillar having a pillar body, each pillar bodybeing such that during an erase operation, the pillar body and theion-implanted well form a single node; a plurality of bitlines and aplurality of wordlines, the plurality of source lines being parallel tothe plurality of bitlines and comprising a respective source line foreach bitline.

Another broad aspect of the present disclosure provides a method formaking a flash memory device, comprising: forming a cell substrate in away that a silicon surface has regions with n-type and p-type silicon;depositing cell stacking layers having gate material and interlayerdielectric; and patterning word lines.

Another broad aspect of the present disclosure provides a device havinga vertical structure of cells and diffused source lines running in adirection perpendicular to word lines, the device comprising cellpillars and a substrate having an ion-implanted well, wherein the cellpillars are formed so that during an erase operation, each cell pillarand the ion-implanted substrate form a single node.

Another broad aspect of the present disclosure provides a methodcomprising: forming diffused source lines; forming a cell stack;performing patterning on the cell stack; wherein forming diffused sourcelines is performed before proceeding with forming the cell stack andperforming patterning.

Another broad aspect of the present disclosure provides a methodcomprising: forming diffused source lines at a same photolithographymask step used to define a location of an alignment mark.

Other aspects and features of the present disclosure will becomeapparent to those ordinarily skilled in the art upon review of thefollowing description of specific embodiments of the present disclosurein conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure will now be described, by way ofexample only, with reference to the attached Figures, wherein:

FIG. 1 illustrates a string (A), page (B) and block (C) in a NAND Flashcell array;

FIG. 2 illustrates a NAND flash cell array structure;

FIG. 3 illustrates a structure of one NAND flash block consisting of 2mprogrammable pages;

FIG. 4 illustrates a NAND flash page structure;

FIG. 5 illustrates a NAND flash cell transistor;

FIG. 6 illustrates a cell threshold voltage distribution for singlelevel cells (1 bit-2 states);

FIG. 7 illustrates a cell threshold voltage distribution for multi-levelcells (2 bit-4 states);

FIG. 8 illustrates an erase operation by Fowler-Nordheim (F-N)tunneling;

FIG. 9 illustrates a program operation by Fowler-Nordheim (F-N)tunneling;

FIG. 10 illustrates a read erased cell (data ‘1’);

FIG. 11 illustrates a read programmed cell (data ‘0’);

FIG. 12 illustrates page read bias conditions;

FIG. 13 illustrates a cell substrate structure;

FIG. 14 illustrates a bias condition during erase for a selected and anunselected block;

FIG. 15 illustrates page program bias conditions;

FIG. 16 illustrates program timing;

FIG. 17 illustrates vertical pillar NAND Flash strings with electricallyisolated pillar bodies;

FIG. 18 illustrates vertical pillar NAND Flash strings with pillarbodies electrically connected to the cell array substrate;

FIG. 19 illustrates cell current flowing in entire page to a singlesource line;

FIG. 20 illustrates a source line manufacturing step;

FIG. 21 illustrates an alternative source line manufacturing step;

FIG. 22 illustrates dopant diffusion between the substrate and the cellbody;

FIG. 23 illustrates a p-well implant manufacturing step;

FIG. 24 illustrates an active/field patterning manufacturing step;

FIG. 25 illustrates a source line formation manufacturing step(embodiment);

FIG. 26 illustrates a cell stack deposition manufacturing step;

FIG. 27 illustrates a pillar hole formation manufacturing step;

FIG. 28 illustrates a cell gate dielectric deposition manufacturingstep;

FIG. 29 illustrates a cell pillar hole second etching manufacturingstep;

FIG. 30 illustrates a cell body fill manufacturing step;

FIG. 31 illustrates cell pillar fill layer details

FIG. 32 illustrates a word line patterning manufacturing step;

FIG. 33 illustrates a String drain n+ implant step and bit lineformation manufacturing step;

FIG. 34 illustrates a p-well implant manufacturing step;

FIG. 35 illustrates a source line formation manufacturing step(embodiment);

FIG. 36 illustrates a cell stack deposition manufacturing step;

FIG. 37 illustrates a pillar hole formation manufacturing step;

FIG. 38 illustrates a cell gate dielectric deposition manufacturingstep;

FIG. 39 illustrates a cell pillar hole 2nd etching manufacturing step;

FIG. 40 illustrates a cell body fill manufacturing step;

FIG. 41 illustrates a word line patterning manufacturing step;

FIG. 42 illustrates a string drain n+ implant step and bit lineformation manufacturing step;

FIG. 43 illustrates an example of a photolithography mask step to definesource lines and location of alignment mark;

FIG. 44 illustrates a further photolithography mask step to finish thealignment mark;

FIG. 45 illustrates a hard mask strip step;

FIG. 46 illustrates a cell stack deposition step;

FIG. 47 illustrates an alignment mark open step;

FIG. 48 illustrates cell current flowing in entire page to separatesource lines; and

FIG. 49 illustrates dopant diffusion from the n-type source line to thecell body in case of tube-shaped cell bodies.

DETAILED DESCRIPTION

Generally, the present disclosure relates to a nonvolatile memorydevice, such as, for example, a flash memory device. The flash memorymay comprise NAND flash memory and other types of flash memories. Ageneral non-limiting example description of NAND Flash memory deviceswill be given in subsequent sections.

The given examples throughout the present disclosure will be shown withthe assumption that junctionless NAND cell transistors consist ofn-channel transistors on p-type substrate. However, the presentdisclosure is not restricted to this case. N and p-type impurity regionsmay be interchanged so as to form p-channel transistors on n-typesubstrate.

The proposed method and apparatus may be applied to, for example:

-   -   1. NAND Flash memory devices with cell strings being located in        a way that cell are stacked in a direction perpendicular to the        chip surface and cell strings are aligned in a way as to form a        pillar vertically to the chip surface.    -   2. The source of the cell string is located below the layers        forming the cells at the bottom of each cell pillar and is        located in the cell array substrate formed as an n-type        diffusion layer.    -   3. When cell transistors (including the string select transistor        and ground select transistor) of a string are turned on, and a        channel is formed in the pillar, the channel of the transistor        most close to the string source (usually the ground select        transistor) is electrically connected to the source of the cell        string and thus the source line.    -   4. Sources of adjacent cell strings are connected with each        other forming a source line consisting of an n-type diffusion        layer.    -   5. The p-type bodies of the pillars are electrically connected        to the underlying p-type substrate of the cell array without a        junction in-between.

Organization of a NAND Flash Memory Cell Array

The basic cell array organization of NAND flash memory devices will bedescribed. FIG. 1 serves as an illustration to describe the termsstring, page and block in a NAND Flash memory device.

A NAND cell string as illustrated in the box “A” of FIG. 1 consists ofat least one string select transistor (SST) which is placed in serieswith the cell transistors and with one terminal (hereinafter referred toas the drain) being connected to the bit line. A NAND cell string alsocontains a certain number of memory cell transistors and at least oneground select transistor which is serially connected between the celltransistors and the source line.

Although in this figure the string consists of 16 cells, the presentdisclosure is not restricted to any specific number of cells per string.The number of cells per string varies, with 4 cells per string, 8 cellsper string, 32 cells per string, 64 cells per string, 128 cells perstring or any other number >1 also being possible embodiments.

Memory cell gates in FIG. 1 are coupled to word lines (commonlyabbreviated WL in embodiment of the present disclosure) 0 to 15. Thegate of the string select transistor (SST) is connected to a stringselect line (SSL) while the drain of the string select transistor (SST)is connected to a bit line (BL). The gate of the ground selecttransistor (GST) is connected to a ground select line (GSL) while thesource of the ground select transistor (GST) is connected to a sourceline (SL or CSL).

To specify a direction within a string, the direction towards the SSL ofa string will be referred to as “drain direction” or “drain side” andthe direction towards the GSL of a string will be referred to as “sourcedirection” or “source side” hereinafter.

The box “B” in FIG. 1 illustrates a common example of a page in a NANDFlash device. A page is the smallest unit addressed by a row address.The smallest unit for which a read or program operation can be performedis also one page. In some common examples one page is identical to allcells connected to one word line. However, other examples also existwhere cells connected to a certain word line are subdivided intomultiple subgroups which thus constitute multiple pages per word line,whereby each one of the multiple pages in one word line has a differentrow address. In the case of multiple bit storage in one physical cell,different bits can belong to different pages although they arephysically located in the same cell transistor and thus connected to thesame word line. Hereinafter, the proposed technique will be describedusing but not be restricted to the example in FIG. 1 where each wordline corresponds to one page.

The box “C” in FIG. 1 illustrates the meaning of a cell block. It isconstituted by the entirety of strings which share the same word lines,string select lines and ground select lines. In the most common examplesof NAND Flash memory devices the smallest unit for which an eraseoperation can be performed is one cell block, which is therefore oftennamed “erase block”.

Assuming that the row address is made of n bits for the block addressand m bits for the page address, FIG. 2 illustrates the cell arraystructure of NAND flash memory. It consists of 2^(n) erase blocks, witheach block subdivided into 2^(m) programmable pages as shown in FIG. 3.

Each page consists of (j+k) bytes (times 8 bits) as shown in FIG. 4. Thepages are further divided into a j-byte data storage region (data field)with a separate k-byte area (spare field). The k-byte area is typicallyused for error management functions.

-   -   1 page=(j+k) bytes    -   1 block=2^(m) pages=(j+k) bytes*2^(m)    -   Total memory array size=2^(n) blocks=(j+k) bytes*^(2m+n)

Basic Cell Operation of Erase, Program and Read

Examples of erase, program and read operations in a NAND flash memoryare described as follows. The structure of a typical NAND Flash cell isillustrated in FIG. 5. NAND flash cell transistors store information bytrapping electrons in a floating node either by a technology commonlyreferred to as “floating gate” or by a technology commonly referred toas “charge trap”. The electrons trapped in the floating node of a celltransistor modify the threshold voltage of this cell transistor todifferent levels depending on the data (0 or 1) stored in the cell. Thethreshold voltage of the cell transistor influences the channelresistance of the cell transistor.

In some common examples memory cells store two logic states; data ‘1’and data ‘0’ and each memory cell corresponded to one bit. In this casethe flash memory cell can have one of two threshold voltagescorresponding to data ‘1’ and data ‘0’. The cell threshold voltagedistribution for these SLC (single level cells) is shown in FIG. 6. Insome widely used NAND Flash devices cells can also be programmed to morethan two threshold levels and thus multiple bits can be stored in onephysical cell (see FIG. 7), which are referred to as MLC (multi-levelcells). Even if no explicit reference is made to multiple bit storageexamples of the proposed systems and methods can be applied equally toNAND memory devices with single and multiple bit storage per cell.

Typically a NAND flash memory cell is erased and programmed byFowler-Nordheim (F-N) tunneling. During an erase operation, the top polyelectrode (i.e. top gate) of the cell is biased to Vss (ground) whilethe cell body is biased to an erase voltage V_erase and the source anddrain of the cell are floated (in the case that the source and the drainconsist of N+ diffusion layers they are automatically biased to V_erasedue to junction-forward-bias from the cell body to the source/drain).With this erase bias condition, trapped electrons (charge) in thefloating poly (i.e. floating gate) are emitted uniformly to thesubstrate through the tunnel oxide as shown in FIG. 8. The cellthreshold voltage (Vth) of the erased cell becomes negative as alsoshown in FIG. 8. In other words, the erased cell transistor is in anon-state with a gate bias Vg of 0V.

During program operation, on the contrary, the top poly (i.e. top gate)of the cell is biased to a program voltage Vpgm while the substrate,source and drain of the cell are biased to Vss (ground). More precisely,the high Vpgm voltage (e.g. 20V) induces a channel under the tunneloxide. Since this channel is electrically connected to the source anddrain which are tied to Vss=0V, the channel voltage Vch is also tied toground. By the difference in voltage Vpgm−Vch, electrons from thechannel are uniformly injected to the floating poly (floating gate)through the tunnel oxide as shown in FIG. 9.

The cell threshold voltage Vth of the programmed cell becomes positiveas also shown in FIG. 9. In other words, the programmed cell is turnedoff with a gate bias Vg of 0V).

In order to read cell data, the gate and drain of the selected cells arebiased to 0V and a read voltage Vrd, respectively while the source ofthe selected cells are set to 0V. If the cell is in an erased state asshown in FIG. 10, the erased cell has a negative threshold voltage andthus a cell current (Icell) from the drain to the source flows under thegiven bias condition. Similarly if the cell is in a programmed state asshown in FIG. 11, the programmed cell has a positive threshold voltageand there is no cell current from the drain to the source under thegiven bias condition. Finally a sense amplifier connected to each bitline senses and latches cell data; an erased cell (on-cell) is sensed asdata ‘1’ and a programmed cell (off-cell) is sensed as data ‘0’.

Page Read

FIG. 12 shows bias conditions during page read operations. The selectedword line is set to 0V while unselected word lines, SSL, and GSL arebiased to a read pass voltage Vread that is sufficiently high to renderunselected cell transistors conductive regardless of their programmedstate (i.e. cell Vth). The common source line CSL is set to ground. Withread bias conditions, the Vth of the selected cell determines cellcurrent Icell. This cell current Icell is sensed by a bit line senseamplifier in a page buffer. An entire page is read in parallel. In orderfor a read operation to work without disturbance, the source line needsto be solidly tied to ground without any modification by the cellcurrents.

Block Erase in NAND Flash

The bias conditions of various nodes in the cell array including thecell body will be described. A detailed description can also be foundin, for example, U.S. Pat. No. 5,473,563 in which non-volatilesemiconductor memories using arrays of cell units include memorytransistor divided into several memory blocks, each having certainnumber of cell units, with erasable selectable memory blocks. FIG. 13shows the structure of the cell array substrate for most widely usedNAND Flash devices. The cell body is formed by a pocket p-well which isisolated from the p-substrate of chip.

FIG. 14 and Table 1 show typical bias conditions during eraseoperations. The cell body is biased to the erase voltage V_erase whilethe floating bit lines and the source lines (SL) in the selected blockare clamped to V_erase-0.6V through the S/D diodes of the SSL and GSLtransistors. At the same time all word lines in the selected block arebiased to 0V while the string select line (SSL) and the ground selectline (GSL) are biased to erase voltage V_erase. Therefore all cellswithin the selected block are erased by F-N tunneling as described inprevious section. Because the substrate of the cells is biased to erasevoltage V_erase and the source/drain/substrate of cells in the selectedblock are electrically connected, the erase operation must occur on ablock basis. In other words, the minimum erasable array size is a block.

Because of the block basis erase operations, erasure of memory cells inunselected blocks sharing the same cell substrate must be prevented(i.e. erase inhibit). For this purpose the self-boosting erase inhibitscheme has been proposed (e.g., U.S. Pat. No. 5,473,563). To preventerasure of memory cells in unselected blocks, all word lines inunselected blocks are floated during erase operations. Therefore floatedword lines in unselected blocks are boosted to nearly erase voltageV_erase by capacitive coupling between the substrate and word lines (theexact value depending on the coupling ratio the word line level liesaround 90% of V_erase when the substrate of the cell array goes toV_erase. The boosted voltage of word lines in unselected blocks reducesthe electric field between the cell substrate and word lines. As aresult erasure of memory cells in unselected blocks is prevented.

TABLE 1 Bias Conditions during Erase Selected Block Unselected BlockBitlines (BL) Clamped to V_erase-0.6 V Clamped to V_erase-0.6 V StringSelect Boosted to approx. 90% of Boosted to approx. 90% of Line (SSL)V_erase V_erase Wordlines 0 V Boosted to approx. 90% of (WL0~WL15)V_erase Ground Select Boosted to approx. 90% of Boosted to approx. 90%of Line (GSL) V_erase V_erase Source Clamped to V_erase-0.6 V Clamped toV_erase-0.6 V Line (SL) Cell body V_erase V_erase

The mentioned bias conditions describe the most widely used schemes.Variations do exist for specific cell technologies where the cell bodyis an electrically isolated node and its potential is raised duringpotential by GIDL charge injection from the source line and where thesource line is not left floating but is raised to an erase voltage levelV_erase.

Page Program and Program Inhibit

The program operation of a single cell was described in a previoussection, where it was described that a high program voltage Vpgm isapplied to the control gate, whereas the channel voltage Vch under thetunnel oxide of the cell transistor is tied to the ground level Vss.Cells which are intended to be programmed during program operation willbe referred to as “program cells” or “selected cells” hereinafter.

A string to which a cell to be programmed during program operationbelongs will be referred to as a “selected string” or “program string”,and bit lines which are connected to such strings will be referred to as“program bit lines” or “selected bit lines” hereinafter. Strings ofwhich the cells should not be programmed during the program operationwill be referred to as “unselected strings” or “program inhibitedstrings”, and bit lines which are connected to such strings will bereferred to as “program inhibit bit lines” or “unselected bit lines”hereinafter.

Expanding the program scheme to entire pages and strings which belong toone block, a common method will be described for supplying the neededbias conditions for cell programming during program operation.Furthermore, a method referred to as channel self-boosting programinhibit will be described which ensures that no cells are programmedinadvertently during program operation which are connected to selectedword lines and whose control gates are therefore biased with Vpgm butwhich belong to unselected strings and are not intended to beprogrammed.

FIG. 15 depicts known page program bias conditions (see, e.g., Kang-DeogSuh et al., “A 3.3 V 32 Mb NAND Flash Memory with Incremental Step PulseProgramming Scheme,” IEEE J Solid-State Circuits, vol. 30, no. 11, pp.1149-1156, April 1995). FIG. 15 is used to describe the program andprogram inhibit operations of NAND Flash devices. Hereby the so-calledchannel self-boosting program inhibit scheme which is used in someexisting techniques will be described. The needed bias conditions areapplied to selected cells as follows in the most common technique:

The program voltage Vpgm is applied to the control gate of a selectedcell through the word line to which the program cell is connected. Forbrevity this word line will be referred to as “selected word line”hereinafter. The SSL transistor of the selected string is turned on withVcc applied to the SSL and the GSL transistor turned off. The bit linevoltage for a selected cell to be programmed with data “0” is set toVss=0 V. Thus the ground level Vss is supplied to the channel of theselected cell through the program bit line and the SSL to which thisparticular string is connected to and through the serially connectedcell transistors on the drain side of the selected cell between theselected cell and the SSL. These “drain side” cells are in a turned onstate with Vpass applied to their control gates to be able to pass onthe channel voltage Vss. For another reason related to program inhibitdescribed below, source side cells are also turned on with Vpass appliedto their control gates in most existing techniques. A continuous channelis formed from the bit line to the selected cell (and beyond) with achannel voltage Vch of 0V. When the program voltage Vpgm is applied tothe gate of a selected cell, the large potential difference between gateand channel level Vch results in F-N tunneling of electrons into thefloating gate.

For program inhibited cells (i.e. cells which should stay in an erasedstate with data ‘1’) and program inhibited strings the connected programinhibit bit line is set to Vcc. For program inhibit, the bit line levelof Vcc initially precharges the associated channel through the turned onSSL transistor, the gate of which is biased also with Vcc as it isconnected to the same SSL which also turns on the SSL transistors ofprogram strings. The coupled channel voltage rises, and once the channelvoltage reaches Vcc-Vth (SSL) the SSL transistor shuts off and thestring channel of the program inhibit string becomes a floating node.

Once the word lines of the unit string rise during program operation(selected word line to the program voltage Vpgm and unselected wordlines to the pass voltage Vpass), the series capacitances through thecontrol gate, floating gate, channel, and bulk are coupled and thechannel potential Vch is boosted automatically beyond the prechargelevel of Vcc-Vth(SSL). Hereby the word lines on the source side of theselected cell are also raised to Vpass to participate in thechannel-boosting. It was shown previously [e.g., Kang-Deog Suh et al.,“A 3.3 V 32 Mb NAND Flash Memory with Incremental Step Pulse ProgrammingScheme,” IEEE J Solid-State Circuits, vol. 30, no. 11, pp. 1149-1156,April 1995] that the floating channel voltage rises to approximately 80%of the gate voltage. Thus the channel voltages of program inhibitedcells are boosted to approximately 8 V in the case that Vpgm˜15.5-20 Vand Vpass˜10 V are applied to the control gates. This high channelvoltage prevents F-N tunneling in the program inhibited cells.

FIG. 16 shows an example of timing the voltages during programoperation. Numerous variations of this program timing scheme existincluding the application of multiple pulses for Vpgm and Vpass.Although the embodiments of the present disclosure will be describedusing the program timing given in FIG. 16, they are not restricted toany program timing scheme in particular.

Vertical Cell Transistors

This embodiment deals with a specific type of NAND Flash transistorcells, where NAND cell strings are stacked in a direction which runsperpendicular to the chip surface. A vertical unit which comprises acell string will be called “pillar” hereinafter. Examples of these kindsof NAND cells have been described in the following references:

-   -   H. Tanaka, “Bit Cost Scalable Technology with Punch and Plug        Process for Ultra High Density Flash Memory”, 2007 Symposium on        VLSI Technology Digest of Technical Papers    -   Jaehoon Jang et al. “Vertical Cell Array using TCAT (Terabit        Cell Array Transistor) Technology for Ultra High Density NAND        Flash Memory”, 2009 Symposium on VLSI Technology Digest of        Technical Papers    -   Yoohyun Noh et al. “A New Metal Control Gate Last Process (MCGL        process) for High Performance DC-SF (Dual Control gate with        Surrounding Floating gate) 3D NAND Flash Memory”, 2012 Symposium        on VLSI Technology Digest of Technical Papers

FIG. 17 and FIG. 18 show two different examples of such verticallyaligned NAND cell strings. Although not a necessary requirement for thisembodiment, in the most common embodiments the gates and the word lineswrap around the pillar bodies thus forming a gate-all-around structure.In the given examples as well as throughout the description it isassumed that NAND cell transistors consist of n-channel transistors onp-type substrate. However, N and p-type impurities may be interchangedso as to form p-channel transistors on n-type substrate.

Care has to be taken for the definition of the terms “body”, “substrate”or “well”, etc. to avoid any ambiguities. In planar NAND devices thebodies of the cells are identical to the overall underlying cellsubstrate as it consists of the same pocket p-well. In vertical devicesthis is not necessarily so. The bodies of the cell transistors arelocated within the individual pillars. These are spatially in locationsdifferent from the overall cell array substrate and may or may not beelectrically connected with the cell array substrate. Therefore thebodies of the cell pillars may or may not be identical to the cell“body” or “substrate” that is formed by the underlying p-well. In thepresent description, whenever the body within a cell pillar is meantthis will be named “pillar body”. Whenever the substrate common to theentirety of the cell array is meant this will be designated by the term“cell substrate” or “cell array substrate”. In addition, where anion-implanted well of a substrate is referred to, this can be a pocketp-well, or it can be the entire substrate.

Two different kinds of cell structures exist for the described verticalcell structures. The first kind is shown in FIG. 17. The p-type pillarbody is isolated from the p-type cell substrate by an n+ type diffusionlayer which forms the source line of the cell strings. This type ofvertical NAND device is not the subject of the proposed technique. Thesecond kind is shown in FIG. 18, where the p-type pillar body isconnected to the p-type cell substrate without any junction in between.The cell pillars thus have an opening at the bottom where the pillarbody and cell array substrate, which are of the same impurity type, areelectrically connected with each other.

The mechanism through which the bodies of the cell pillars are biasedduring erase operation is one aspect of the difference in the cellstructure. In the first in FIG. 17 case where the p-substrate of thecell array is not in direct electrical connection with the pillarbodies, GIDL mechanisms have been proposed to charge the pillar bodies.This will not be described in detail here.

As another aspect of the difference in the cell structures the n-typeand p-type regions at the bottom of the cell pillars take a differentshape in the two cases, respectively. In the case where the pillar bodyis isolated from the cell substrate the bottom diffusion layer may beallowed to form a continuous n-type region if viewed from above withoutp-type regions. In this case the n-type region may extend in bothdirections, the bit line and the word line direction. This is indicatedin the schematic on the right side of FIG. 17 by the line connectionbetween adjacent source lines.

In the case where the pillar body is not isolated from the cellsubstrate certain restrictions apply to the exact shape of the n-typeregion, as there always have to exist regions under the cell pillarswhich are not n-type to ensure the p-type pillar bodies are electricallyconnected to the p-type cell substrate, but the n-type channels of thepillar transistors are connected to the n-type regions of the substrate.Those restrictions may result in difficulties connecting source linesarbitrary directions.

The proposed technique applies to the second of the mentioned caseswhere p-type pillar bodies are connected to the p-type substrate of thecell array without any junction in-between.

The application provides a method of manufacturing vertically stackedNAND Flash memory cells of a type as described in FIG. 18 with adiffused n+ source line where the cell body is of the same impurity typeas and electrically connected to the chip substrate and not separated bya pn junction, and corresponding devices. The proposed manufacturingscheme improves weak points present with prior manufacturing schemes.

A first weak point to be improved is present with some priormanufacturing schemes described in the next section. The problem isknown as source line bounce which is caused by high cell currents andhigh source line resistances. It is of foremost importance that thesource line maintains the voltage applied to it during read or writeoperations throughout the entirety of the cell array. There are,however, practical obstacles to it.

-   -   As the source line consists of diffusion layers in some common        vertical NAND Flash devices the sheet resistance may be up to        several ohms/square, which can add up through the entire length        of the source line to a resistance of several hundred kiloohms        and more, even if a silicidation process is applied to reduce        the resistance.    -   If the resistance of the source lines is high it is        disadvantageous if all cells of the same page are connected to        the same source line. See for example FIG. 19 where the source        lines run in the same direction as the word lines. In a read        operation a cell current flows through all cell strings which        are part of a selected block, e.g. 64K cells for 8 KB sized        pages. Assuming a sheet resistance of a few ohms/square for the        silicided diffusion layer and cell currents around 100        nanoampere, the source line bounce can add up to be in the volt        range in the middle of the source line if no other measures are        taken.

Thus it seems not desirable that the source lines run in the word linedirection as all the currents of cells belonging to the same page willcrowd on the same source line. However, the source line running in adirection parallel to the word line is a natural outcome of some priormanufacturing schemes.

A second weak point is present with some other manufacturing schemesdescribed in the next section. It is related to the fact that althoughthe manufacturing process may be aimed at forming a structure such thatthe pillar body is open at the bottom and connected to the p-typesubstrate of the chip, in some specific cell structure n-type dopantdiffusion from the source line may isolate the open path between thecell pillar body and the chip substrate (see example 2 in the nextsection).

An existing method of forming diffused source lines is shown FIG. 20.This is the underlying method for forming vertical NAND Flash cells asdescribed in U.S. Pat. No. 8,203,211 B2, U.S. Pat. No. 8,278,170 B2.

The source line is formed by impurity implantation and diffusion afteretching the word lines, using the word line patterns as a mask. It canbe easily seen that the direction of the source lines in the word linedirection follows naturally from the manufacturing method.

To reduce the source line resistance additional measures have beenproposed as in “U.S. Pat. No. 8,203,211B2, Nonvolatile memory deviceshaving a three dimensional structure”, where strapping regions areintroduced where the diffused source lines are connected by diffuseddummy regions in the bit line direction. However, since these regionscannot be used as memory cells, this solution comes with an increase incell array size.

In another scheme as in Y. Noh et al., “A New Metal Control Gate LastProcess (MCGL process) for High Performance DC-SF (Dual Control gatewith Surrounding Floating gate) 3D NAND Flash Memory”, 2012 Symposium onVLSI Technology Digest of Technical Papers, p. 19-20, it is proposedthat a blank substrate n+ diffusion layer is formed by blank ionimplantation first. In a later step the pillar contact hole is etchedthrough the blank n+ diffusion layer to establish an electrical contactbetween the cell pillar body and the substrate p-well (FIG. 21).

This scheme may be weak to n-type dopant diffusion from the n+ source tothe cell pillar body, especially for some schemes where the cell pillarbody does not consist of bulk poly-Si which fills the entire hole, butwhere the cell pillar body consists of a thin poly-Si film surrounding adielectric contact filler. N-type dopant diffusion from the substrate n+region may turn the impurity type of the tube type cell body at thebottom region into n-type as shown in FIG. 22, thereby disconnectingcell body from the substrate p-well.

The embodiments described hereinafter will show manufacturing methodsand corresponding devices which:

-   -   1. have as a result that source line diffused regions are        connected in a direction perpendicular to the word lines and        parallel to the bit line direction;    -   2. bodies of cell pillars are electrically connected to the        ion-implanted well and are of the same impurity type;    -   3. cell pillar patterns are aligned to the impurity regions        forming the source lines; and    -   4. are robust to dopant diffusion of unwanted impurity type.

First Embodiment

In a first embodiment a manufacturing method is provided where a sourceline is formed first, cell stacking layers are formed thereafter and thecell pillars are formed in alignment to the previously formed sourcelines. FIG. 23 to FIG. 33 show the step-by-step manufacturing process.It is understood that the shown manufacturing steps include only commonsemiconductor processes. The left of each Figure shows a top view of aregion of a cell area, the middle of each Figure shows a vertical cut ina region of a cell area in the direction of the word lines, the right ofeach Figure shows a vertical cut in a region of a cell area in thedirection of the bit lines.

In the first embodiment the source line itself is not only formed bydiffusion but by patterning, (including etching) as well. The patterningstep used to perform patterning in the cell area is also used to createa photo alignment mark in a photo alignment mark region, outside thecell area. Further details of the photo alignment mark are describedbelow in the context of the second embodiment.

In the manufacturing step shown in FIG. 23 a p-well in the cell regionis formed by ion implantation. The ion implanted p-well is indicated at101. Although not shown the p-well in the cell region may be a pocketp-well which is separated from the p-well substrate of the chipperiphery circuitry.

In the next step as shown in FIG. 24 stripe-shaped active/field regionsare formed by a standard semiconductor manufacturing process, forexample the so-called STI (shallow trench isolation) process. Thetrenches run in the direction of the bit lines. Hereby stripe-shapedtrenches (fields) are formed and filled with a dielectric material 102,for example silicon oxide. In some embodiments this Active/fieldpatterning process is performed at the same time as the Active/fieldpatterning of the rest of the chip circuitry such that no separateprocess exists for the cell area.

In the next step as shown in FIG. 25 ion implantation and annealing areperformed to form n-type conductive regions 103 in the active regions,but not in the non-conducting field regions 102. In some embodimentsthis source/drain formation process is performed at the same time as thesource/drain formation process of the rest of the chip circuitry suchthat no separate process exists for the cell area. In some embodimentsthe order of the steps in FIG. 24 and FIG. 25 may be interchanged suchthat the n+ diffusion process is performed first and the active/fieldpatterning process thereafter.

In the next step as shown in FIG. 26 alternating non-conductivedielectric layers 104 a-104 c and conducting layers 105 a-105 c aredeposited. From the conducting layers 105 a-105 c the gates of the celltransistors are formed in later steps. The non-conducting layers areinterlayer dielectric layers between the cell transistor gates. Theconducting layers may be for example n+ or p+ doped poly-Si. The lowestnon-conducting layers 104 a may be of lower thickness (for examplearound 10 nm) than the intermediate non-conducting layers 104 b, whichmay be around 20 nm thick. The lowest non-conducting layer 104 a may beof a different material, for example undoped silicon oxide, from theintermediate non-conducting layers 104 b, which may be for example dopedsilicon oxide. In some embodiments the lowest non-conducting layer 104 amay be formed through an oxidization process of the silicon substrateand not through a layer deposition process. The top non-conducting layer104 c may be of a higher thickness, for example 40 nm, than theintermediate non-conducting layers 104 b. The highest non-conductinglayer 104 c may be of a different material, for example silicon nitride,from the intermediate non-conducting layers 104 b. Although only 4conductive layers are shown it is understood that as in most known NANDFlash devices, the number of layers corresponds to the numbers oftransistors in NAND Flash strings which may be a number higher than 4,for example the number of intermediate conductive layers may be 64.

In the next step as shown in FIG. 27 holes 106 are etched in a firstetching step such that pillar holes are formed in a regular row andcolumn pattern and such that the holes are formed on regions where thestripe shaped field patterns 102 were formed in an earlier step. This isachieved by aligned this step to an alignment mark which was formedduring the active/field patterning step. Furthermore the holes 106 areetched through the cell stack layers 104 a-104 c and 105 a-105 c but notthrough the field dielectric 102.

In the next step as shown in 28 the gate dielectric layers 107 aredeposited. Although not shown it is understood that the gate dielectriclayer 107 may for example be a multi-layer structure consisting of thetunnel dielectric, the charge trap layer and a coupling dielectric as insome common NAND Flash cells. The tunnel dielectric may be siliconoxide, the charge trap layer may be silicon nitride and the couplingdielectric may be silicon oxide. Although shown as a single step, thedielectric formation step may be multiple steps to form differentdielectric layers at the lowest 105 a, the intermediate 105 b and thehighest 105 c regions.

In the next step in FIG. 29 a second etch step is performed whichextends the existing pillar holes (106 of FIG. 27) through the fielddielectric to produce holes 108 shown in FIG. 29 and thereby expose theunderlying p-well substrate.

In some embodiments, the etch step of FIG. 29 is divided into two steps.These would include a first etch step removes the gate dielectric layer7 except from the vertical inner sidewalls of the pillar holes. A secondetch step then extends the existing pillar holes (106 of FIG. 27)through the field dielectric to produce holes 108 shown in FIG. 29 andexposes the underlying p-well substrate. The first and second etch stepmay be either separate or combined into one etch step.

In the next step in FIG. 30, a pillar body 109 is formed in each pillarhole. For the purpose of this example, it is assumed that the pillarbody is p-doped silicon such that the pillar body and the p-type ionimplanted well form a single node during an erase operation of theresulting NAND flash memory. They form a single node in the sense thatif one is charged, the other is charged as well; there is no junction inbetween.

FIG. 31 shows an example of a cross section of a multilayer filled cellpillar employed in some embodiments. The pillar is surrounded by thegate material 105. The outermost layers comprise the gate dielectrics107 a-107 c. Going in a direction from the outermost to the innermostlayers, the filling layers consist of a tunnel dielectric 107 a whichmay be silicon oxide, a charge trap layer 107 b which may be siliconnitride, a coupling layer 107 c which may be silicon oxide, thetransistor body 109 a which may be undoped poly silicon and a dielectricfiller 109 b which may be silicon oxide. In another embodiment there isno dielectric filler 109 b, but the silicon body 109 a fills all theremaining innermost part of the pillar.

In the next step as shown in FIG. 32 the word lines of all verticallystacked layers are etched at the same single photolithography mask stepto form alternating fin patterns 110 and slit patterns 111.

In the next steps as shown in FIG. 33 the slits 111 of FIG. 32 arefilled with an isolating dielectric such as for example silicon oxide asindicated at 112. In a subsequent step the topmost part of the cellpillars is doped such as to form an n+ diffusion layer 113 which is atthe same time the drain of the NAND Cell pillar. In a subsequent stepthe bit lines 114 are formed with a standard metallization process suchas to form electrical connections to the string drains 113.

Second Embodiment

FIG. 34 to FIG. 47 show a step-by-step manufacturing process in thesecond embodiment.

The step in FIG. 34 is identical to the step in FIG. 23 in the firstembodiment. Contrary to the first embodiment there is no active/fieldpatterning process that includes etching in the cell region. In otherwords, the step of FIG. 24 is not performed.

In the next step as shown in 35 an n+ source implant process isperformed at a photolithography mask step such as to form astripe-shaped diffusion layer 300 of n-type and a stripe-shaped 301diffusion layer of p-type. Contrary to the first embodiment nopatterning of the active substrate through etching takes place.

In the second embodiment this step comprises a photolithography maskstep used to define the n-type source lines that at the same timedefines a location of a photo alignment mark through patterning,including etching, outside the cell area. The formation of thestripe-shaped diffusion layer 300 and the alignment mark will bedescribed separately later.

The next step in FIG. 36 is identical to the step in FIG. 26 in thefirst embodiment.

The next step in FIG. 37 is identical to the step in FIG. 27 in thefirst embodiment. As in the first embodiment the cell pillar holes 106are aligned to the substrate n+ diffusion regions in a way that theholes are formed on regions which are of p-type diffusion. However,contrary to the first embodiment there do not exist any active/fieldpatterns in the cell region. The alignment of this photolithography maskstep is performed using an alignment mark, the formation of which isdescribed later.

The step in FIG. 38 is identical to the step in FIG. 28 in the firstembodiment.

The step in FIG. 39 is identical to the step in FIG. 29 in the firstembodiment. However, contrary to the first embodiment the second etchstep does not extend the pillar hole through a field dielectric layerbut through to the substrate silicon. In some embodiments, the pillarholes are extended a defined distance into the substrate silicon.

The remaining steps in FIG. 40 to FIG. 42 are identical to the steps inFIG. 30 to FIG. 33 in the first embodiment.

A method is provided to align the pillar patterns to the underlying nand p-type diffusion regions. In this embodiment, etchedphotolithography alignment marks are formed. This is shown in FIG. 43 toFIG. 47. A first photolithography mask step is shown in FIG. 43. A hardmask material which has a different etch selectivity from the substratesilicon is deposited. The material may be for example silicon oxide. InFIG. 43, a photoresist is applied and then the hard mask is etched atthe cell region and at the alignment mark region such as to expose thesubstrate silicon at the regions which are to be doped with the n+ typeion implantation to define the location of the source lines and thealignment mark. Subsequently, ion implantation to form the bottom sourceline is performed at the cell region and at the alignment mark regionalike.

FIG. 44 shows a subsequent photolithography mask step where an etchedpattern is formed in the substrate at the alignment mark but not in thecell area.

At the next step as shown in FIG. 45 the hard mask material is strippedby a selective etch process.

In subsequent steps as shown in FIG. 46 and FIG. 47 which are identicalto the cell formation steps described before the cell stack layers (104a-104 c, 105 a-105 c in the previous descriptions) are deposited on topof the diffused cell regions and the alignment marks.

There are two different ways this photo alignment mark can be utilizedfor the alignment in the subsequent cell pillar patterning process. Inthe first method simply the height difference of the alignment mark canbe directly used. In the second method an alignment mark open step canbe performed as depicted in FIG. 47 to remove the cell stack layers onlyfrom the photo alignment mark region and thus to enhance the visibilityof the alignment mark.

In both embodiments described so far, the source line patternsconsisting of impurity implanted (and diffused) regions are formedbefore the stacking and patterning of the cell stack layers takes place.This gives the freedom to align the source line patterns so as to run ina direction perpendicular to the word lines.

The advantage of connecting source lines in the bit line directioninstead of in the word line direction can be seen in FIG. 48. This waythe cell current of each cell string flows into separate source linesand thus source line bouncing is reduced.

In both embodiments, the location of the photolithography alignment markis defined at the same photolithography mask step as the source lineformation. This way it is ensured that alignment of cell pillar patternsrelative to the source line patterns can take place in a later step.

As another advantage, the proposed scheme with stripe-shaped impurityregions may be more robust than the scheme in Y. Noh et al., “A NewMetal Control Gate Last Process (MCGL process) for High PerformanceDC-SF (Dual Control gate with Surrounding Floating gate) 3D NAND FlashMemory”, 2012 Symposium on VLSI Technology Digest of Technical Papers,p. 19-20, where the bottom source part of the cell pillar is surroundedby an n-type impurity region (see FIG. 49).

OTHER EXAMPLES

Although the given examples throughout the description are shown withthe assumption that NAND cell transistors consist of n-channeltransistors on p-type substrate and source lines consisting of n-typeregions, the proposed technique is not restricted to this case. Thepolarity of the impurity types may be exchanged. N and p-type impurityregions may be interchanged so as to form p-channel transistors onn-type substrate and p-type source lines.

Although not mentioned explicitly, subsequent silicidation of the n-typediffusion layer or any other process to reduce the sheet resistance ofthe source line layer may be used.

The conductive layers 105 a-105 c which form the gate and word linematerial may be some metal such as tungsten instead of doped poly-Si.

Although the second embodiment is described in a way that the etching ofthe hard mask occurs before the impurity implant steps, it is equallypossible to first perform the impurity implant steps through the hardmask material using the photoresist mask and afterwards to etch the hardmask to pattern the alignment marks, as long as source line impurityimplant and the location of the alignment mark is defined in the samephotolithography mask step.

Although the descriptions of the embodiments suggest that source linesrun in one direction (the bit line direction) only, n-type source linestrapping regions may be additionally interconnected in the word linedirection. Although for the effectiveness of the proposed technique itis sufficient that the source lines patterns being connected in adirection perpendicular to the word lines, additional strapping regionswhich provide additional connections between regions of the sameimpurity type are also possible in connection with the presentdisclosure. In other embodiments the source lines are not necessarilyparallel to the bit lines.

Although the proposed technique does not rely on any subsequent impurityimplant after the formation of the cell stack, additional impurityimplant at later steps is not excluded.

In the embodiments described herein, it is for the most part assumedthat the pillar body is p-type silicon and the ion-implanted well isp-type silicon. More generally, in any of the embodiments describedherein, the pillar body may instead be intrinsic (undoped) silicon or belightly n-doped silicon, if in any of these cases it is still true thatduring an erase operation, the pillar body and the p-type ion-implantedwell form a single node, with no junction in between.

Although the embodiments of the present disclosure have been describedusing the example of n-channel transistors on bodies of p-typeconductivity, the present disclosure also applies for examples where allconductivity types are reversed so as to form p-channel transistors onbodies of n-type conductivity.

In some embodiments, the pillar hole is filled with a multi-layerstructure consisting of that material that will form the connection tothe ion-implanted well and a dielectric filler like silicon oxide whichfills the innermost part of the hole.

The description contains numerous references to a photolithography maskstep. A photolithography mask step can be viewed as the sum of allsemiconductor chip manufacturing steps which are used to create onephotoresist pattern, wherein as a result of this photoresist pattern,the photoresist covers some lateral regions in the chip but not other,AND the sum of all semiconductor chip manufacturing steps (e.g. etching,ion implant) which are applied on some lateral regions but not on otherlateral regions in the chip, whereby the distinction between thesedifferent lateral regions is done using the same photoresist pattern forall these manufacturing steps, and whereby this photoresist pattern wascreated exactly once using a photolithography mask.

For example if a photoresist pattern is created using a photolithographymask, and a first manufacturing step is performed using this photoresistpattern and a second manufacturing step is performed using the samephotoresist pattern (without removing and recreating the photoresistpattern in between the two manufacturing steps), it is said that thefirst manufacturing step and the second manufacturing step occur at thesame photolithography mask step.

On the other hand, for example if a first photoresist pattern is createdusing a photolithography mask, and a first manufacturing step isperformed using the first photoresist pattern, the first photoresistpattern is removed, a second photoresist pattern is created using adifferent or the same photolithography mask, a second manufacturing stepis performed using the second photoresist pattern, it is said that thefirst and the second manufacturing steps do not occur at the samephotolithography mask step.

It is noted that a photolithography mask is a device which is used (andreused) within a photolithography tool, whenever a certain kind ofphotoresist pattern needs to be created. It is not to be confused withphotoresist masks (or patterns) or hard masks, which are patterns on thewafer.

In the embodiments described above, the device elements and circuits areconnected to each other as shown in the figures for the sake ofsimplicity. In practical applications these devices, elements circuits,etc., may be connected directly to each other or indirectly throughother devices elements, circuits, etc. Thus, in an actual configuration,the elements, circuits and devices are coupled either directly orindirectly with each other.

The above-described embodiments of the present disclosure are intendedto be examples only. Alterations, modifications and variations may beeffected to the particular embodiments by those of skill in the artwithout departing from the scope of the present disclosure, which isdefined solely by the claims appended hereto.

1. A method for manufacturing a NAND Flash memory device with a vertical cell stack structure, the method comprising: forming a first n-type conductive region and a second n-type conductive region in a cell substrate, each of the first and the second n-type conductive regions being arranged in a first horizontal axis parallel to an upper surface of the cell substrate, and being physically disconnected form each other; forming a stack of layers comprising gate layers and insulating layers over the cell substrate including the steps of: depositing a bottom insulating layer, and depositing a bottom gate layer over the bottom insulating layer, and alternatingly depositing intermediate insulating layers and intermediate gate layers over the bottom gate layer, and depositing a top gate layer over an uppermost one of the intermediate insulating layers, and depositing a top insulating layer over the top gate layers, the top insulating layer comprising at least one of a different material and a different thickness than the intermediate insulating layers; forming a first cell pillar including the steps of: a first etching step wherein a first pillar hole is formed through the stack of layers; and depositing a gate dielectric layer, the gate dielectric layer covering sidewalls and a bottom of the first pillar hole; a second etching step for extending the first pillar hole into the cell substrate by removing a bottom portion of the gate dielectric layer and etching a first recess into the cell substrate; filling the first pillar hole with a first pillar body, the first pillar body comprising a poly silicon film surrounding a dielectric filler, the first pillar body extending into the first recess, such that a sidewall of the first pillar body is in contact with the first n-type conductive region, and forming a drain at an upper portion of the first pillar body; and forming a first bit line above the stack of layers, the first bit line connected to the drain and extending in a second horizontal axis parallel to the upper surface of the cell substrate and perpendicular to the first horizontal axis.
 2. The method of claim 1, wherein the dielectric filler comprises silicon oxide.
 3. The method of claim 1, wherein the top insulating layer comprises silicon nitride, and wherein the intermediate insulating layers comprise silicon oxide.
 4. The method of claim 1, wherein the top insulating layer is formed to comprise a greater thickness than the thickness of each of the intermediate insulating layers.
 5. The method of claim 1, wherein a portion of the dielectric filler is formed in the recess in the cell substrate such that the cell substrate surrounds the portion of the dielectric filler.
 6. The method of claim 1, wherein the gate layers are formed of poly silicon material.
 7. The method of claim 1, further comprising: forming a first trench and a second trench in the cell substrate, such that the first n-type conductive region is adjoined to the first trench and the second trench; and filling each of the first and the second trenches with a dielectric material.
 8. The method of claim 1, wherein the second n-type conductive region is separated from the first n-type conductive region by the second trench.
 9. The method of claim 1, wherein the drain is formed as an n-type diffusion region.
 10. The method of claim 1, wherein after the second etching step at least a portion of a sidewall of the gate dielectric layer is substantially flush with a sidewall of the first recess.
 11. The method of claim 1, wherein the bottom insulating layer is formed of at least one of a different material and a different thickness than each of the intermediate insulating layers.
 12. The method of claim 1, wherein the gate dielectric layer comprises a first sublayer, a second sublayer, and a third sublayer.
 13. The method of claim 12, wherein the first sublayer comprises a tunnel dielectric surrounding the poly silicon film.
 14. The method of claim 13, wherein the second sublayer comprises a charge trap layer surrounding the tunnel dielectric, and the third sublayer comprises a coupling layer surrounding the tunnel dielectric.
 15. The method of claim 14, wherein the tunnel dielectric comprises silicon oxide, and wherein the charge trap layer comprises silicon nitride.
 16. The method of claim 13, wherein the coupling layer comprises silicon oxide.
 17. The method of claim 1, further comprising etching word lines by etching a slit pattern having a plurality of slits through the stack of layers, each slit of the plurality of slits exposing a portion of the cell substrate, wherein the step of etching word lines occurs later than the step of forming the first n-type conductive region and the second n-type conductive region.
 18. The method of claim 1, wherein forming the first n-type conductive region and the second n-type conductive region comprises performing an n+ diffusion process, and wherein a first trench and a second trench are formed in the cell substrate after the n+ diffusion process is performed.
 19. The method of claim 1, further comprising forming a second cell pillar simultaneously with the first cell pillar, the second cell pillar comprising a second pillar body, wherein of the first n-type conductive region and the second n-type conductive region only the second n-type conductive region is physically connected to the second pillar body. 